Stabilized Laser Source with Very High Relative Feedback and Narrow Bandwidth

Abstract

This invention relates to the stabilization of a laser source used in optoelectronics, specifically a source comprising a semiconductor laser diode (1). Such laser sources are often used as so-called pump lasers for fiber amplifiers in the field of optical communication, erbium-doped fiber amplifiers being a prominent example. Such lasers are usually designed to provide a narrow bandwidth optical radiation with a stable power output in a given frequency band. The present invention now concerns such a laser source using external reflector means, preferably consisting of one or more appropriately designed fiber Bragg gratings (9), providing very high relative feedback with an extremely narrow bandwidth, combined with a very long external cavity encompassing about 100 modes or more and an extremely low front facet (2) reflectivity of the laser diode. This stabilizes the laser source extremely well in its operation, without the need for an active temperature stabilizing element.

Description

FIELD OF THE INVENTION[0001]This invention relates to the stabilization of a laser source, specifically a semiconductor laser diode of the type commonly used in opto-electronics, mostly as so-called pump lasers for fiber amplifiers in the field of optical communication. Erbium-doped fiber amplifiers are a prominent example using such laser diodes. Usually, such laser sources are designed to provide a relatively narrow-bandwidth optical radiation with a stable power output in a given frequency band. In particular, the invention relates to a laser using external reflector means providing very high relative feedback with a narrower bandwidth compared to conventional devices and in addition extremely low noise operation, even without an active temperature stabilizing element. Another advantage of the invention is the reduction of spectral distortions, in the case that polarization maintaining fiber is used. Such a laser source can also be used in different applications like frequency do...

AI Technical Summary

Benefits of technology

[0002]Semiconductor laser diodes of the type mentioned above have, for example, become important components in the technology of optical communication, particularly because such laser diodes can be used for amplifying optical signals immediately by optical means. This allows the design of all-optical fiber communication systems, avoiding complicated conversions of the signals to be transmitted. The latter improves speed as well as reliability within such communication systems.

[0003]In one kind of optical fiber communication systems, the laser diodes are used for pumping erbium-doped fiber amplifiers, so-called EDFAs, which have been described in various patents and publications known to the person skilled in the art. An example of some technical significance is 980 nm lasers with a power output of 100 mW or more, which wavelength matches the 980 nm erbium absorption line and thus achieves a low-noise amplification. InGaAs laser diodes have been found to serve this purpose well and are used today in significant numbers. However, the invention is not limited to InGaAs laser diodes, but may also be used for other types as explained below.

[0004]Generally, laser diode pump sources used in fiber amplifier applications operate in the single transversal and vertical mode for efficient coupling into single-mode fibers and are mostly multiple longitudinal mode lasers, i.e. Fabry-Perot (FP) lasers. Three main types of laser diodes are typically used for erbium amplifiers, corresponding to the absorption wavelengths of erbium: InGaAsP and multiquantum-well InGaAs lasers are used at 1480 nm; strained quantum-well InGaAs lasers at 980 nm; and GaAlAs lasers at 820 nm.

[0005]Some fiber amplifier configurations require a defined polarization state of the light coming from the pump laser. Hence, depending on the application, pump sources are build with a polarization maintaining fiber to serve this particular requirement. Alternatively and less costly, a non-polarization maintaining fiber may be used, with similar pump source performance.

[0006]One of the problems occurring when using semiconductor laser diodes for the above purpose is their wavelength and power output instability which, though small, still affects the amplification sufficiently that there is motivation to look for a solution to the problem.

[0007]This problem is already addressed in U.S. Pat. No. 5,563,732 by Erdogan et al., entitled “Laser Pumping of Erbium Amplifier”, which describes the stabilization of a pump laser of the type described above by use of a Bragg grating in front of the laser diode. This grating provides an “external cavity” between the front facet of the laser diode and the grating in addition to the “laser cavity” or “active cavity” of the laser diode. The laser's emission spectrum is stabilized by the reflection from the grating. The grating is formed inside the guided-mode region of the optical fiber at a certain distance from the laser diode. Such a fiber Bragg grating is a periodic (or aperiodic) structure of refractive index variations in or near the guided-mode portion of the optical fiber, which variations are reflecting light of a certain wavelength propagating along the fiber. The grating's peak-reflectivities and reflection bandwidths determine the amount of light reflected back into the laser diode.

Problems solved by technology

One of the problems occurring when using semiconductor laser diodes for the above purpose is their wavelength and power output instability which, though small, still affects the amplification sufficiently that there is motivation to look for a solution to the problem.

Additionally, low-frequency power fluctuations seem to decrease by the effect of induced high-frequency multi-mode operation.

None of the above prior art addresses solutions for high power (i.e. >100 mW) laser sources, capable of stable operation without using an active temperature stabilizing element.

The need for TEC's contributes significantly to the complexity and cost of a laser source.

A less known and undiscussed problem is that of spectral distortions which may occur when a polarization-maintaining fiber is used for typical laser-grating configurations in fiber amplifier systems.

However, slight residual reflections act as small pertubations to the dominant mode field, influencing—and sometimes deteriorating—the performance of the laser source.

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